Constant magnification lens for vision system camera

ABSTRACT

This invention provides a lens assembly for a vision system, allowing for a constant magnification at various focal distances. The lens assembly resides movably/adjustably along the optical axis relative to the sensor. In an embodiment, the lens assembly includes a fixed rear lens and a front lens that is moved mechanically to focus the object image on the image sensor. The lens assembly can alternatively include a liquid lens that is controlled to adjust magnification with respect to a fixed front lens so as to maintain a constant system magnification. The liquid lens resides between the (fixed) front lens assembly and the image sensor and can be controlled to focus the image onto the image sensor.

RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application Ser.No. 61/745,927, filed Dec. 26, 2012, entitled CONSTANT MAGNIFICATIONLENS FOR VISION SYSTEM CAMERA, the entire disclosure of which is hereinincorporated by reference.

FIELD OF THE INVENTION

This invention relates to machine vision systems and more particularlyto optics for use in handheld symbology readers, and methods for use ofsuch optics.

BACKGROUND OF THE INVENTION

Vision systems that perform measurement, inspection, alignment ofobjects and/or decoding of symbology (e.g. bar codes) are used in a widerange of applications and industries. These systems are based around theuse of an image sensor, which acquires images (typically grayscale orcolor, and in one, two or three dimensions) of the subject or object,and processes these acquired images using an on-board or remote,interconnected vision system processor. The processor generally includesboth processing hardware and non-transitory computer-readable programinstructions that perform one or more vision system processes togenerate a desired output based upon the image's processed information.This image information is typically provided within an array of imagepixels each having various colors and/or intensities. In the example ofa symbology (barcode) reader, the user or automated process acquires animage of an object that is believed to contain one or more barcodes. Theimage is processed to identify barcode features, which are then decodedby a decoding process and/or processor obtain the inherent alphanumericdata represented by the code. In other types of vision systems, variousvision system tools (e.g. edge detectors, calipers, blob analysis) areemployed by the system processor to detect edges and other features thatallow for recognition of object features, and the determination ofdesired information based upon these features—for example whether theobject is defective or whether it is properly aligned.

In a vision system, a key component is the vision system cameraassembly. The camera assembly includes a lens (optics) and an imager (or“sensor”) that provides the array of image pixel information. The visionsystem processor, as described above, receives the pixel data from theimager/sensor and processes it to derive useful vision systeminformation about the imaged scene and/or object. The vision systemprocessor and related components (e.g. data memory, decoders, etc.) canbe provided within the camera assembly's housing or enclosure, or someor all of these components can be mounted remotely (e.g. within a PC, orother remote, self-contained processing system), and linked by a wiredor wireless interconnect. Likewise, the camera assembly can include anon-board (internal) illuminator that typically surrounds the lens,and/or another illumination arrangement that provides light to theimaged scene.

In some vision system cameras, it is desirable to provide an automaticfocus (“auto-focus”) capability. Many auto-focus arrangements rely uponelectromechanical actuation to move a fixed lens, while othersincreasingly rely upon other forms of varioptic lens designs, such as aso-called liquid-lens.

In the particular field of symbology reading using sensor-based visionsystems, a common reader arrangement employs a handheld unit that isdirected at an object containing a symbol (e.g. a 1D or 2D barcode).Such handheld systems are commonly employed to track inventory, forexample in a warehouse or factory floor. In such environments, thedistance between a symbol and the reader can be highly variable, as someobjects can reside relatively close to a user, while others are disposedat a distance (e.g. an object located on a high shelf). While a readymay include a conventional auto-focus mechanism to allow it to generatea sharp image of both the close object and the far object, the symbol inthe far object can appear small relative to the overall field of viewgiven this long focal distance as the opening angle of the optics is toolarge. As such, the small size of the symbol in the overall image mayrender it difficult to properly decode due to lack of sufficientresolution when compared to the overall field that is captured by thesensor (i.e. the feature of interest/symbol is too small at distance).

It is therefore desirable to provide a vision system camera assemblythat can more effectively resolve a symbol or other feature of interestat both short focal distances and long focal distance. This cameraassembly should be adaptable to a handheld device and/or to afixed-mount device.

SUMMARY OF THE INVENTION

This invention overcomes disadvantages of the prior art by providing alens assembly for a vision system, such as a handheld symbology reader,which allows for a constant magnification at both short and long focaldistances. The lens assembly resides movably and/or adjustably along theoptical axis at a predetermined distance from the image sensor (locatedgenerally perpendicular to the axis). In an illustrative embodiment, thelens assembly consists of two lenses L1 and L2 separated from each otheralong an optical axis. In embodiments, L1 and L2 can be represented bygroups of lenses. The two lenses L1 and L2 define respective focallengths f1 and f2. The lenses satisfy the following relationships: (a)the focal points of L1 and L2 coincide; (b) the aperture stop of theassembly is between the back surface of L1 and the focal point of L1;(c) the magnification of the assembly is constant and equal to f2/f1;and (d) the shift in focal position of the assembly is(f1/f2)²*(movement of the assembly along the optical axis). In anembodiment, the lens is moved to a selected position along the opticalaxis, and with respect to the sensor, by an actuator (e.g. a gearedstepper or servo motor). The actuator moves in response to commands fromthe vision processor that employs a conventional or custom auto-focusprocess to resolve a sharp image of the feature of interest (e.g. asymbol) on an object at the prevailing focal distance. The feature ofinterest will appear at approximately the same resolution at each of amaximum and minimum focal distance and at all ranges therebetween. Thesensor's inherent pixel resolution is sufficient at the range ofoperating distances to provide sufficient detail to identify and decodethe symbol.

In an illustrative embodiment, a vision system for acquiring images ofobjects over a range of focal distances within a field of view includesan image sensor operatively connected to a vision processor. A constantmagnification lens assembly, oriented along an optical axis, andincluding a front lens assembly, receives light from a scene andtransmits the light to the image sensor. The front lens assembly issmaller in area (or associated dimensions) than an area (or associateddimensions) of the field of view, making of a practical and relativelycompact package. The constant magnification lens assembly also includesa rear lens assembly. The front lens assembly and the rear lens assemblyare arranged in a fixed spatial relationship therebetween.Illustratively, the front lens assembly and the rear lens assembly areconstructed and arranged so that (a) a focal point of the front lensassembly and a focal point of the rear lens assembly coincide, (b) anaperture stop of the constant magnification lens assembly is between aback surface of the front lens assembly and a focal point of the frontlens assembly, (c) a magnification of the assembly is constant and equalto a focal length (f2) of the rear lens assembly/a focal length (f1) ofthe front lens assembly, and (d) the shift in focal position of theassembly is (f1/f2)²*(movement of the constant magnification lensassembly along the optical axis). This arrangement (i.e. item (b))allows the front lens assembly to define an area and/or dimensions thatare smaller than those of the imaged field of view. The front lensassembly and the rear lens assembly can be mounted in a barrel that ismoved toward and away from the image sensor by an actuator responsive toa focus process. Alternatively, the constant magnification lens assemblyand its components can be fixed with respect to the camera body/frame,and the sensor assembly (or a portion thereof containing the sensor) canbe moved toward and away from (along the optical axis) the constantmagnification lens assembly by an appropriate actuator in response tothe focus process.

In an illustrative embodiment, a vision system for acquiring images ofan object over a range of focal distances within a field of viewcomprises an image sensor operatively connected to a vision processor. Aconstant magnification lens assembly is oriented along an optical axisthat receives light from a scene and transmits the light to the imagesensor. The constant magnification lens assembly includes a liquid lensassembly oriented between the image sensor and a front lens assembly.This lens assembly can be based upon the use of at least two iso-densityliquids that vary interaction based upon the principle of electrowetting, or the lens can include an actuator that changes the shape of aliquid-filled membrane. The front lens assembly comprises one or morefixed lenses, and the (rear) liquid lens assembly including an interfacethat employs input electrical energy to vary a magnification m2 of theliquid lens assembly. A controller selectively adjusts the magnificationm2 of the liquid lens assembly to maintain focus on the object at aconstant system magnification M at each focal distance of the range offocal distances. Illustratively, the front lens assembly and the rearlens assembly are constructed and arranged so that (a) a focal point ofthe front lens assembly and a front principal plane of the liquid lensassembly coincide, and (b) the magnification is constant and equal to aratio between the distance (d2) from the liquid lens assembly to theimage sensor and the distance (d1) between the front lens assembly andthe liquid lens assembly. The controller is also arranged to iterativelyadjust the magnification m2 of the liquid lens assembly until a desiredfocus at the constant system magnification M is provided.

Illustratively, in any embodiment herein, the front lens assembly andthe liquid (or rear) lens assembly are constructed and arranged so that(a) a focal point of the front lens assembly and the front principalplane of the rear lens assembly coincide (d1=f1), (b) an aperture stopof the constant magnification lens assembly is located between a backsurface of the front lens assembly and a focal point of the front lensassembly, (c) the magnification is constant and equal to a ratio betweenthe distance (d2) from the liquid lens assembly to the image sensor andthe distance (d1) between the front lens assembly and the liquid lensassembly.

In a further embodiment, a method for acquiring images of an object overa range of focal distances within a field of view includes the steps ofproviding an image sensor operatively connected to a vision processorand a constant magnification lens assembly oriented along an opticalaxis that receives light from a scene and transmits the light to theimage sensor. The constant magnification lens assembly includes a frontlens assembly. The constant magnification lens assembly is iterativelyadjusted until the object achieves a desired focus at the image sensor.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention description below refers to the accompanying drawings, ofwhich:

FIG. 1 is a diagram of a handheld symbology reader, and associated dataprocessing and storage system, employing a constant magnification lensassembly according to an illustrative embodiment to acquire images ofobjects at a short focal distance and a long focal distance;

FIG. 2 is an image of a scene acquired by the image sensor of the readerof FIG. 1, showing resolution of a feature of interest at a short focaldistance;

FIG. 3 is an image of a scene acquired by the image sensor of the readerof FIG. 1, showing resolution of a feature of interest at acomparatively long focal distance;

FIG. 4 is a cross section of the lens assembly for use in the reader ofFIG. 1, according to an illustrative embodiment, showing the relativeposition of the assembly along the optical axis with respect to theimage sensor for a relatively short focal distance;

FIG. 5 is a cross section of the lens assembly shown in FIG. 4, in whichthe relative position of the assembly along the optical axis withrespect to the image sensor is set for a longer focal distance;

FIG. 6 is a diagram of a lens focus mechanism and associated processorsfor processing image data and controlling focus of the lens assembly inthe reader of FIG. 1;

FIG. 7 is a flow diagram of an exemplary focus process using the lensassembly in the reader of FIG. 1;

FIG. 8 is a diagram of a lens assembly for a constant magnificationvision system including a liquid lens assembly according to anillustrative embodiment;

FIGS. 9 and 10 are thin-line ray-trace diagrams of the lens assembly forthe constant magnification vision system of FIG. 8, shown maintainingconstant magnification on an exemplary object at a nearer andmore-distant positioning, respectively, by controlling magnification ofthe liquid lens assembly;

FIG. 11 is a diagram of a lens assembly for a constant magnificationvision system including a liquid lens assembly and an aperture stoplocated between the liquid lens assembly and a front lens assemblyaccording to another illustrative embodiment.

DETAILED DESCRIPTION

I. General Considerations

FIG. 1 shows a vision system 100, which includes at least one symbologyreader 110 that can be handheld as shown, or fixed in a position withrespect to an imaged scene. The reader can define any acceptablehousing, including the depicted main body 112 and grip 114. In thisembodiment, the reader includes a front window 116, which can include anexternal and/or internal illumination system (illuminator). Theilluminator can comprise any arrangement and/or combination of lightingelements in any acceptable arrangement. In this embodiment, and by wayof example, light elements (e.g. high-output LEDs) 120, 122 are employedand allow for differing color/wavelength, angle and/or intensity ofillumination. The illuminator can include conventional aiming LEDs (notshown) that project a beam onto a field of view to ensure that featuresof interest (e.g. barcodes or other symbols, also termed “IDs”) areproperly and fully imaged. The reader 110 can include an indicator andinterface panel 130, located at the rear of the body 112 in thisembodiment. This panel can include on/off and other switches as well aslights to indicate a “good” or “failed” symbol read (i.e. success orfailure in reading/decoding the symbol). The grip 114 can include one ormore trigger buttons 132 that trigger illumination and image captureamong other functions, such as toggling of aiming LEDs. The reader alsoincludes one or more processing circuits, memory and the like, that arecollectively shown (in phantom) as a vision processor 136. Thisprocessor performs various image processing, and image datahandling/storage functions. Illustratively, the processor 136 receivescaptured image frame rate in the form of color or grayscale pixels(among other formats) from the image sensor (also shown in phantom). Theprocessor searches for ID features (or other features of interest) inthe image, and then passes appropriate data to a decoding process thatgenerates codes from the ID features. These codes are stored and/orpassed via a communication link (which can be wired, or wireless asshown) 140 to a receiver 142 that is interconnected via a network orother link with a data processing and storage system 144. This system144 can comprise a conventional server or PC running appropriateapplications for handling and storing code data transmitted from thereader 110. Such applications and the architecture of the system 144should be clear to those of skill in the art.

The reader also includes a lens assembly 150 (shown in phantom behindwindow 116) that provides for a constant magnification over a range offocal distances. By way of example, an object O1 having a symbol S1 isimaged by the reader 110 with the lens 150 focusing upon a field of viewFOV1 in which the symbol S1 occupies a relatively prominent place/scaletherewithin. This scale is sufficient to allow sufficient detail for anacceptable ID reading. The focal distance D1 along optical axis OA1 iswithin an operating range of least approximately 350 mm, and for thepurposes of the example is at a distance of approximately 500 mm.Likewise, the reader 110 can be focused (as shown in phantom) on anotherobject O2 located at a significantly shorter focal distance D2 alongoptical axis OA2 that, for the purposes of the example, is approximately50 mm. Notably, using the constant magnification lens assembly 150, inaccordance with an illustrative embodiment, the scale of the secondsymbol S2 within the associated field of view FOV2 is approximately thesame as that of S1 and FOV1. Hence, regardless of distance within apredetermined distance range, the size of the field of view and symboltherewithin remains the same, allowing for sufficient detail to obtain agood read.

With reference to FIGS. 2 and 3, the principle of constant magnificationis further illustrated by respective exemplary images 200 and 300 thatsimulate the appearance of images acquired, respectively, at focaldistances of approximately 50 mm and 500 mm. The field of view of eachimage 200, 300 is defined by the outside edges of the depicted image.Using a constant magnification arrangement, both images should ideallypresent approximately the same boundaries relative to the scene.Likewise, each exemplary symbol region 210, 310 appears to be relativelysimilar in size with respect to the field of view, allowing sufficientdetail for the processor to find and decode the symbol.

II. Constant Magnification with Mechanically Driven Lens

Reference is now made to FIGS. 4 and 5, which show the constantmagnification lens assembly 150 in further detail, according to anillustrative embodiment. The assembly 150 consists of a front lens L1(with a focal length f1) and a rear lens L2 (with a focal length f2)aligned along the optical axis OA with respect to a sensor 410, whichtypically defines an image plane perpendicular to the axis OA. Thelenses L1, L2 in this embodiment are positioned at a fixed distance SLwith respect to each other with in barrel 420, or other supportingstructure that maintains their relative alignment and spacing. Each ofthe two lenses L1, L2 is particularly designed to establish a set ofrelationships that ensure constant magnification over a range of focaldistances. More particularly, these relationships are as follows:

-   -   (a) the focal points of L1 and L2 coincide at the depicted plane        (d1=f1+f2);    -   (b) the aperture stop of the assembly (AS) is between the back        surface of L1 and the focal point of L1;    -   (c) the magnification of the assembly is constant and equal to        f2/f1; and    -   (d) the shift in focal position of the assembly is        (f1/f2)²*(movement of the assembly along the optical axis).

Note that the placement of the aperture stop at a position defined initem (b) above is advantageous in that the size of the front lens can besmaller in diameter, area, etc., than an area, length, width, etc., theimaged object and associated field of view. Conversely, placement of theaperture elsewhere (e.g. at focal point f1), could necessitate use of afront lens approximately the size of the desired field of view—forexample, in the manner of a telecentric lens. Such a large lens istypically disadvantageous where size and placement constraints exist.

Note, it is also expressly contemplated that the depicted lenses L1and/or L2 can be defined by group(s) of lenses having similar or thesame optical power as a single lens element. In various embodiments,such groups of lenses can provide improved correction of opticalaberrations relative to single, discrete lens elements. Thus as usedherein the term “lens” should be taken broadly to include an arrangementof a plurality of discrete lenses.

One of skill in the art of lens design should understand theconstruction of a lens assembly that satisfies the above relationships(a)-(d). In an embodiment, the value f2/f1 is approximately 0.1, butother ratios are expressly contemplated. Illustratively, both groups oflenses L1 and L2 define a positive optical power. By way of example,lens L1 can define a focal length between approximately 30 and 60millimeters and lens L2 can define a focal length between approximately6 and 10 millimeters. As shown, at a relatively short focal distance FD1(FIG. 3), the arrangement of lenses L1, L2 in the assembly 150 defines aray pattern 430 that diverges at a steeper angle for a given distanceDS1 between the sensor 410 and the rear lens 410. This more divergentpattern defines a focus on a field of view 440 that is a desired sizefor appropriately imaging a feature of interest (e.g. a symbol/ID)therein.

To achieve a similarly sized field of view 540 (FIG. 5) to the field 440at a longer focal distance FD2, the distance DS2 between rear lens L2and sensor 410 is shortened with respect to the above distance DS1. Thusthe ray pattern 530 is less divergent. Note at both focal distance theaperture stop AS is the same. Thus, by appropriately moving the distancebetween the rear lens and the sensor, the optical system can be broughtinto focus on a similarly sized field of view at a wide range of focaldistances.

With reference now to FIG. 6, an exemplary focus mechanism 610 formoving the lens assembly 150 along the optical axis OA, toward and awayfrom the sensor 138 (shown mounted on an associated imager circuit boardthat defines the base of a sensor assembly 630) is depicted. Note thatthe depicted mechanism 610 is exemplary of a wide variety of possiblefocus-adjustment mechanisms, others implementations of which should beclear to those of skill in the art. In this exemplary embodiment, themechanism 610 includes an actuator in the form of a motor (e.g. a servoor stepper motor) 620 having appropriate torque, and where desired, gearreduction to rotate a pinion gear 622 in each of opposing rotationaldirections (double arrow 624). The motor rotates a large gear 626 havingan inner perimeter that is enmeshed with the outer surface of the lensassembly by mating threads 628 that can include a relatively shallowpitch. As the gear 626 rotates based upon the drive of the motor, itcauses the lens assembly to move toward or away (double arrow 631) fromthe sensor 630 assembly and associated sensor image plane. In thisembodiment, an anti-rotation pin 632, which is fixed to the housing body112, engages an axially-aligned slot 634 in the lens assembly 150 toprevent rotation of the lens. In this manner rotation of the gear isfully translated intro linear motion of the lens assembly along the axisOA. A variety of alternate anti-rotation arrangement can be employed inthis implementation. Alternatively the lens can be rotated and a fixed,threaded base (substituting for a rotating gear 626) can be used togenerate linear translation in the lens assembly 150 with respect to thesensor. In further embodiments, the lens can be fixed and the sensor canbe provided on an axially moving base. In general, the system definesrelative motion between the lens assembly 150 and sensor/image plane(138).

The axial position of the lens assembly 150 is determined by the properfocus of the projected image on the sensor 138. In an embodiment, theon-board vision processor 136 includes a focus process 640 that canemploy conventional techniques to determine when an image comes intosharp focus. For example, the contrast fall-off at edges in acquiredimages can be employed. In the focus process 640, the constantmagnification lens assembly 150 is moved (in its entirety as a fixedfront lens L1, rear lens L2 and aperture stop AS) by the motor 620through a multiplicity of position steps, and the process 640 determinesthe best focus position based upon certain metrics in the acquiredimages. Other techniques for focusing the lens are expresslycontemplated—for example, sweeping through lens positions or employing adistance sensor and/or range finder can be employed to determine thedistance to the object/imaged scene and move the lend to a predeterminedsetting. The lens position setting can be based, for example, on aformula or look-up table that uses the sensed distance to determine alens position setting.

With further reference to FIG. 6, it is expressly contemplated that theconstant magnificent lens assembly 150 can be fixed with respect tobody/frame of the camera. In such embodiments, the sensor assembly 630(or a portion thereof (e.g. the sensor 138)) can be moved along theoptical axis OA toward and away from the fixed-position constantmagnification lens assembly 150. An appropriate sensor actuator (SA)(shown in phantom as box 648) can be used to move the sensor assembly630 as indicated by the associated double arrow. Any acceptableactuation mechanism can be used including (but not limited to)electrically powered worm drives, gear drives and/or linear motors.Focus processes, as described generally above (e.g., stepping, sweeping,sensing distance, etc.), can be used to set the appropriate sensorposition with respect to the fixed, constant magnification lensassembly.

III. Constant Magnification Focus Process

With brief reference to FIG. 7, an illustrative procedure 700 foradjusting focus of the constant magnification lens assembly is shown.This procedure 700 is a simplified example of any acceptable procedurefor adjusting focus during setup and/or runtime. In step 710, the sensoracquires one or more image frames of the scene and transmits these imageframes to the vision processor. Among other processes, the processorperforms the focus process (640 in FIG. 6) to determine whether theimage is sufficiently resolved (step 720). If the image is sufficientlyresolved (for example, a symbol can be decoded), then decision step 730allows focus to be set at the current setting (step 740). If focus isunacceptable, or worse than that achieved in a previous adjustment ofthe lens assembly, then the focus process readjusts the lens focus in apredetermined direction over an adjustment increment (step 750).Selection of the predetermined direction form movement of the lensassembly can be based upon a prediction as to which direction willimprove focus, or it can be based upon an analysis of the imageindicating which direction will achieve better focus. If focus is worsethan the previous adjustment, then the direction of adjustment in thenext cycle is reversed accordingly. While adjustment can occur inincrements, using an iterative process, it is contemplated that a largeradjustment can be made initially based upon a determination as to how“out-of-focus” the image is, and then smaller-sized adjustments can bemade until a final focus is achieved. Again, this procedure (700) isexemplary of a wide range of focus-adjustment procedures and/ortechniques that should be clear to those of skill in the art.

IV. Constant Magnification using Liquid Lens

An exemplary lens configuration that can be desirable in certain visionsystem applications is a so-called liquid lens assembly. One form ofcommercially available liquid lens, available, for example fromVarioptic of France, uses two iso-density liquids—oil is an insulatorwhile water is a conductor—and the principle (phenomenon) of electrowetting to vary the optical power setting of the lens. On exampleprovides an 18-diopter (1/focal length) variable range of optical power.The variation of voltage passed through the lens by surroundingcircuitry leads to a change of curvature of the liquid-liquid interface,which in turn leads to a change of the focal length of the lens. Somesignificant advantages in the use of a liquid lens are the lens'ruggedness (it is free of mechanical moving parts), its fast responsetimes, its relatively good optical quality, and its low powerconsumption and size. The use of a liquid lens can desirably simplifyinstallation, setup and maintenance of the vision system by eliminatingthe need to manually touch the lens. Relative to other autofocusmechanisms, the liquid lens has extremely fast response times. It isalso ideal for applications with reading distances that change fromobject-to-object (surface-to-surface) or during the changeover from thereading of one object to another object.

A recent development in liquid lens technology is available fromOptotune AG of Switzerland. This lens utilizes a movable membranecovering a liquid reservoir to vary its focal distance. This lensadvantageously provides a larger aperture than competing designs andoperates faster. The focal length/distance of an optical systememploying liquid lens technology can be varied within a predeterminedrange (e.g. 20 diopters) based upon the setting of the liquid lenselement. This setting is varied by applying force to the perimeter ofthe membrane using electromagnetic actuation in accordance with knowntechniques.

FIG. 8 depicts a generalized lens system configuration 800 for a visionsystem (See FIG. 1) including a liquid lens element L2L oriented alongan optical axis OAL. In this illustrative arrangement, the overall lenssystem 800 consists of at least two lenses or two groups of lenses. Thefirst (front) group L1L has a fixed optical power, the second (rear)group L2L consists of, or includes, a liquid lens element with variableoptical power. Note that the front lens assembly and the (rear) liquidlens assembly are arranged at a fixed spatial relationship along theoptical axis OAL in this embodiment. This liquid lens L2L is driven by adriver or similar processor 810 that provides current to the lens toadjust its focus based upon a lens focus process 820. A complete rangeof exemplary parameters for the system are provided below. If anexemplary object 830 is placed at a distance Sobj (in this example,approximately 20.710 mm) from the optical plane of the first lens(group) L1L with optical power A1, this lens will project an image at adistance:

S_2=Sobj/(A1*Sobj−1)   (Eq. 1)

By way of non-limiting example, the distance d1 between the respectiveoptical planes of lenses L1L and L2L, and the distance d2 between theoptical plane of the liquid lens L2L and image sensor 840 can be fixed.Liquid lens L2L thereby projects this intermediate image onto the sensor840 if the liquid lens' optical power A_LL is equal to (set to):

A_LL=1/(S_2−d1)+1/d2   (Eq. 2)

The geometrical magnification m1 of the first lens (group) is equal to:

m1=1/(A1*S_obj−1)   (Eq. 3)

and the magnification m2 of the second lens (group) is equal to:

m2=1/(A_LL*(S_2−d1)−1)   (Eq. 4)

Now the liquid lens L2L is placed into the back focal point of lens L1L,this expression can also be written as d1=1/A1.

Substituting this into the equations (2) and (4), the totalmagnification M of this system reduces to:

M=m1*m2=d2/d1   (Eq. 5)

where M is the system magnification, m1 is the magnification of thefixed lens group/assembly, m2 is the magnification of the liquid lensgroup/assembly, d1 is the distance between the respective optical planesof lenses L1L and L2L, and d2 is the distance between the optical planeof the liquid lens L2L and image sensor 840.

Thus, this arrangement produces a constant magnification that is free ofdependence on (independent of) the object distance. FIGS. 9 and 10 eachrespectively show an exemplary thin-lens ray-trace though the lensarrangement 800 of the system for two different object distances S3(closer to the optical plane of L1L) and S4 (further from the opticalplane of L11). The focal distance F2L and F2L′ of the liquid lens L2L isappropriately adjusted by the driver 810 and process 820 to project thefocused image at constant magnification M throughout the anticipatedrange of object distances Sobj.

Thus, by varying the optical power of L2L (m2), the value of M can bemaintained at a predetermined level over varying distances of objectfrom the system (Sobj). The value for m2 can be set using a variety oftechniques. The above-described focus process 700 can be used to set m2.That is, the power of the lens L2L can be adjusted incrementally(iteratively) until appropriate focus for the selected constant valuefor M is achieved for the object 830 at a given distance.

Reference is now made to FIG. 11, which shows a constant magnificationlens system 1100 with a liquid lens assembly L2L according to a furtherembodiment. Elements that are similar in structure and/or function asthose described above with reference to FIGS. 8-10 have been providedwith like reference numbers. Illustratively, an aperture stop ASL (seealso the description above for mechanically moved lenses) can bepositioned between the front lens assembly L1L and the rear, liquid lensassembly L2L. The position of this aperture stop ASL along the opticalaxis OAL defines the size of the lenses (groups/assemblies) L1L and L2L.If the aperture stop ASL is positioned toward the front lens assemblyL1L, then the size (diameter) of the liquid lens assembly L2L should beincreased. If the aperture stop ASL is, conversely, positioned towardthe liquid lens assembly L2L, then the size (diameter) of the front lensassembly L1L should be increased. Currently, commercially availableliquid lenses are generally available in smaller diameters, so theaperture stop is generally positioned placed nearer to the liquid lens,or the liquid lens assembly (itself) acts as the aperture stop in thesystem.

Some generalized parameters for an operational example of a constantmagnification lens assembly employing a membrane-type liquid lens areshown and described in the Table as follows:

Focal distance of Lens 1 f1 = 100 mm Closest object distance: S_near =−200 mm Largest object distance S_far = −400 mm Intermediate Near ImageS2 S2_near = 200 mm Intermediate FAR*Image S2 S2_far = 133.333 mmDistance between lenses d1 = f1 mm L2L to sensor distance d2 = 20 mmFocal Length of L2L for Near object f_ll_near = 16.667 mm Optical Powerof L2L (Diopter)@Near A_ll_near = 60 diopter Focal Length of L2L for FARobject f_ll_far = 12.5 mm Optical Power of L2L (Diopter)@Far A_ll_Far =80 diopter Required Optical Power Range of L2L R = 20 diopter (Diopter)Magnification at NEAR distance m1_near = −1 m2_near = 0.2 M_near = m1*m2= −0.2 Magnification at FAR distance m1_far = −0.333 m2_far = 0.6 M_far= m1*m2 = −0.2

It should be clear that the vision system with constant magnificationlens described herein advantageously allows for acquisition of images ofan area of interest at a wide range of focal distances with adequatedetail and relatively straightforward adjustment of the lens assembly.This increases acquisition speed as the distance changes betweenobjects, rendering the system highly suited to handheld vision systemsand to fixed-mount vision systems (for example in a moving conveyorline) that can encounter objects of different size and shape (withassociated differences in focal distance).

The foregoing has been a detailed description of illustrativeembodiments of the invention. Various modifications and additions can bemade without departing from the spirit and scope of this invention.Features of each of the various embodiments described above may becombined with features of other described embodiments as appropriate inorder to provide a multiplicity of feature combinations in associatednew embodiments. Furthermore, while the foregoing describes a number ofseparate embodiments of the apparatus and method of the presentinvention, what has been described herein is merely illustrative of theapplication of the principles of the present invention. For example, asused herein various directional and orientation terms such as“vertical”, “horizontal”, “up”, “down”, “bottom”, “top”, “side”,“front”, “rear”, “left”, “right”, and the like are used only as relativeconventions and not as absolute orientations with respect to a fixedcoordinate system, such as gravity. Moreover, as used herein, the terms“process” and/or “processor” should be taken broadly to include avariety of electronic hardware and/or software based functions andcomponents. Moreover, a depicted process or processor can be combinedwith other processes and/or processors or divided into varioussub-processes or processors. Such sub-processes and/or sub-processorscan be variously combined according to embodiments herein. Likewise, itis expressly contemplated that any function, process and/or processorhere herein can be implemented using electronic hardware, softwareconsisting of a non-transitory computer-readable medium of programinstructions, or a combination of hardware and software. Moreover, whilethe lens assembly is shown as a unit with at least two spatially fixedlenses, and an external actuator, it is expressly contemplated thatadditional lenses and/or other optical elements e.g. filters) can beprovided in alternate embodiments. Also, the lenses of the lens assemblycan be individually actuated by separate actuation devices (or a set ofgears linked to a common motor. Additionally, actuation can be achievedby alternative mechanisms, such as a linear motor. Moreover, the lensassembly can be removable and/or include a self-contained actuator thatis linked to the camera by an appropriate link. In embodimentsemploying, for example, a liquid lens element, the positioning of thefixed lens assembly at the front and liquid lens assembly at the rear isillustrative only. Where appropriately sized liquid lens assemblies areavailable, such can be arranged at the front of the assembly, and afixed (or other) lens assembly can be located at the rear (i.e.more-adjacent to the image sensor and more-distant from the imagedobject/scene). Accordingly, this description is meant to be taken onlyby way of example, and not to otherwise limit the scope of thisinvention.

What is claimed is:
 1. A vision system for acquiring images of objects over a range of focal distances within a field of view comprising: an image sensor operatively connected to a vision processor; a constant magnification lens assembly oriented along an optical axis that receives light from a scene and transmits the light to the image sensor, the constant magnification lens assembly including a front lens assembly; and wherein the front lens assembly is smaller in area than an area of the field of view.
 2. The vision system as set forth in claim 1 wherein the constant magnification lens assembly further includes a rear lens assembly and wherein the front lens assembly and the rear lens assembly are arranged in a fixed spatial relationship therebetween.
 3. The vision system as set forth in claim 2 wherein the front lens assembly and the rear lens assembly are constructed and arranged so that (a) a focal point of the front lens assembly and a focal point of the rear lens assembly coincide, (b) an aperture stop of the constant magnification lens assembly is between a back surface of the front lens assembly and a focal point of the front lens assembly, (c) a magnification of the assembly is constant and equal to a focal length (f2) of the rear lens assembly/a focal length (f1) of the front lens assembly, and (d) the shift in focal position of the assembly is (f1/f2)²*(movement of the constant magnification lens assembly along the optical axis).
 4. The vision system as set forth in claim 3 wherein the front lens assembly and the rear lens assembly are mounted in a barrel that is moved toward and away from the image sensor by an actuator responsive to a focus process.
 5. The vision system as set forth in claim 3 wherein the front lens assembly and the rear lens assembly and the aperture stop are each fixed with respect to each other and further comprising an actuator that moves the image sensor toward and away from the constant magnification lens assembly.
 6. The vision system as set forth in claim 2 further comprising a liquid lens assembly oriented between the front lens assembly and the image sensor, wherein the front lens assembly and the rear lens assembly are constructed and arranged so that (a) a focal point of the front lens assembly and the front principal plane of the rear lens assembly coincide (d1=f1), (b) the rear lens assembly has a variable optical power, (c) an aperture stop of the constant magnification lens assembly is located between a back surface of the front lens assembly and a focal point of the front lens assembly, the magnification of the assembly is constant and equal to the ratio between the distance d2 from the rear lens group to the sensor and the distance d1 between the two lens groups.
 7. A vision system for acquiring images of an object over a range of focal distances within a field of view comprising: an image sensor operatively connected to a vision processor; a constant magnification lens assembly oriented along an optical axis that receives light from a scene and transmits the light to the image sensor, the constant magnification lens assembly including a liquid lens assembly oriented between the image sensor and a front lens assembly, the front lens assembly comprising one or more fixed lenses, and the liquid lens assembly including an electrical interface that varies a magnification m2 of the liquid lens assembly, wherein the front lens assembly and the rear lens assembly are arranged in a fixed spatial relationship therebetween; and a controller that selectively adjusts the magnification m2 of the liquid lens assembly to maintain focus on the object at a constant system magnification M at each focal distance of the range of focal distances.
 8. The vision system as set forth in claim 7 wherein the front lens assembly and the rear lens assembly are constructed and arranged so that (a) a focal point of the front lens assembly and a front principal plane of the liquid lens assembly coincide, and (b) the constant magnification lens assembly is constant and equal to a ratio between the distance (d2) from the liquid lens assembly to the image sensor and the distance (d1) between the front lens assembly and the liquid lens assembly.
 9. The vision system as set forth in claim 8 wherein the liquid lens assembly comprises a membrane liquid lens assembly.
 10. The vision system as set forth in claim 9 wherein the electrical interface comprises an actuator that varies a shape of the membrane.
 11. The vision system as set forth in claim 8 wherein the liquid lens assembly comprises at least two iso-density liquids that vary interaction therebetween in accordance with an electro wetting principle in response to a varied electrical input.
 12. The vision system as set forth in claim 7 wherein the controller is arranged to iteratively adjust the magnification m2 of the liquid lens assembly until a desired focus at the constant system magnification M is provided
 13. The vision system as set forth in claim 7 wherein the front lens assembly is smaller in area than an area of the field of view
 14. A method for acquiring images of an object over a range of focal distances within a field of view comprising the steps of: providing an image sensor operatively connected to a vision processor and a constant magnification lens assembly oriented along an optical axis that receives light from a scene and transmits the light to the image sensor, the constant magnification lens assembly including a front lens assembly; and iteratively adjusting the constant magnification lens assembly until the object achieves a desired focus at the image sensor.
 15. The method as set forth in claim 14 further comprising locating a liquid lens assembly oriented between the front lens assembly and the image sensor, wherein the front lens assembly and the rear lens assembly are constructed and arranged so that (a) a focal point of the front lens assembly and the front principal plane of the rear lens assembly coincide (d1=f1), (b) the rear lens assembly has a variable optical power, (c) an aperture stop of the constant magnification lens assembly is located between a back surface of the front lens assembly and a focal point of the front lens assembly, d the magnification of the assembly is constant and equal to the ratio between the distance d2 from the rear lens group to the sensor and the distance d1 between the two lens groups. 